The present invention relates to sampling circuitry and in particular, but not limited to sampling circuitry for use in converting analog signals into digital signals and to related analog to digital conversion circuitry. The invention has particular although not exclusive relevance to an ‘all-digital’ method, and associated apparatus, for converting analog real-valued radio frequency signals into digital signals suitable for implementing an ‘all-digital’ radio receiver, and to radio receiver/transceiver apparatus that implements such a method.
Circuit design engineers are increasingly faced with the often conflicting challenges of providing increased functionality whilst, at the same time reducing both the size of circuit real estate and power consumption. Coupled with short design cycles, the challenges of efficient analog design and the comparative ease with which digital designs can be verified, this has contributed to an increase in the preference for digital design solutions.
As products become more complex and increasingly portable the number of analog-to-digital converters (ADCs) that are required to support such digital solutions increases. Moreover, as the cost of programmable digital devices, such as Field Programmable Gate Arrays (FPGAs) comes down their popularity for implementing such digital solutions increases. However, as FPGA's become more popular, and products become more complex and portable, there is a corresponding increase in the relative importance of mixed-signal circuit integration and, in particular, the integration of ADCs, digital-to-analog converters (DACs), and power circuits.
Digital implementations of DACs, in the form of Sigma-Delta DACs, are now relatively popular. More recently, a digital ADC has also been proposed that uses a Low Voltage Differential Signal (LVDS) input on an FPGA, with low pass resistor-capacitor feedback, to provide low-pass sampling. However, whilst these solutions offer efficient integration of DACs and ADCs with FPGAs the range of frequencies that they can operate with is relatively low (e.g. audio frequency) which acts as a constraint on the applications in which they can be used.
International Patent Application number WO2015GB53126, which published as WO2016063038, discloses a delta-sigma modulator circuit suitable for operation with much higher (radio) frequencies. The sigma-delta modulator circuit is therefore suitable for use in implementing an all-digital radio transmitter system which can be implemented, for example, using an FPGA or application specific integrated circuit (ASIC).
There is, however, a need for an improved circuitry that may be used to digitally implement an ADC that could, for example, be used at higher (e.g. radio) frequencies. Such an ADC would, for example, be useful for implementing an ‘all-digital’ (or at least mostly digital) radio receiver.